1. Field of the Invention
This invention relates generally to vehicle near obstacle detection systems and, more particularly, to vehicle near obstacle detection systems that provide high target range resolution, detection of both stationary and moving objects, and resolution of multiple targets.
2. Description of the Related Art
A frequency modulated, continuous wave (FMCW) radar system is capable of detecting objects even in the absence of relative motion between the object and the system, and can provide a measure of the range to the object even at very close distances. Furthermore, an FMCW system can detect multiple objects and indicate their ranges. Finally, an FMCW system can operate under virtually all environmental conditions. These qualities make an FMCW system desirable for vehicle near obstacle detection systems. Unfortunately, most present FMCW systems require a relatively large radio frequency (RF) bandwidth to detect objects with any great resolution. Government allocation of RF frequencies constrains the bandwidth available for vehicle near obstacle detection systems, and therefore limits the maximum resolution of such systems.
A two-frequency Doppler (2FD) radar system is capable of providing very accurate range and velocity information for an object. This makes the 2FD system desirable for vehicle near obstacle detection systems. Unfortunately, a Doppler system requires relative motion between the system and the object, and therefore is unable to detect objects that are not moving relative to the system. That is, a 2FD system cannot detect objects in the absence of relative motion. A 2FD system also cannot distinguish between multiple objects. Finally, a 2FD system also incorporates the phenomenon of repeat nodes, whereby range to an object can be accurately determined only within a given interval up to a repeat node distance, and thereafter can only be determined as being located within multiples of that distance. For example, if a 2FD system repeat node occurs at 20 feet, the system might not be able to distinguish between an object at 20 feet, 4 inches and an object at 40 feet, 4 inches. Because of the need for relative motion, the system will be unable to provide information each time the vehicle comes to a stop, and for a time thereafter as the system regains information.
From the discussion above, it should be apparent that there is a need for a vehicle near obstacle detection system that provides the accuracy, multiple object, environmental, and stationary obstacle benefits of an FMCW system and the accurate range and velocity benefits of a 2FD system, without being constrained to one system or the other and without being subject to the drawbacks of the particular system. The present invention satisfies this need.